Derivation of Gaussian Probability Distribution: a New Approach
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Applied Mathematics, 2020, 11, 436-446 https://www.scirp.org/journal/am ISSN Online: 2152-7393 ISSN Print: 2152-7385 Derivation of Gaussian Probability Distribution: A New Approach A. T. Adeniran1, O. Faweya2*, T. O. Ogunlade3, K. O. Balogun4 1Department of Statistics, University of Ibadan, Ibadan, Nigeria 2Department of Statistics, Ekiti State University (EKSU), Ado-Ekiti, Nigeria 3Department of Mathematics, Ekiti State University (EKSU), Ado-Ekiti, Nigeria 4Department of Statistics, Federal School of Statistics, Ibadan, Nigeria How to cite this paper: Adeniran, A.T., Abstract Faweya, O., Ogunlade, T.O. and Balogun, K.O. (2020) Derivation of Gaussian Proba- The famous de Moivre’s Laplace limit theorem proved the probability density bility Distribution: A New Approach. Ap- function of Gaussian distribution from binomial probability mass function plied Mathematics, 11, 436-446. under specified conditions. De Moivre’s Laplace approach is cumbersome as https://doi.org/10.4236/am.2020.116031 it relies heavily on many lemmas and theorems. This paper invented an al- Received: March 17, 2020 ternative and less rigorous method of deriving Gaussian distribution from Accepted: May 29, 2020 basic random experiment conditional on some assumptions. Published: June 1, 2020 Keywords Copyright © 2020 by author(s) and Scientific Research Publishing Inc. De Moivres Laplace Limit Theorem, Binomial Probability Mass Function, This work is licensed under the Creative Gaussian Distribution, Random Experiment Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access 1. Introduction A well celebrated, fundamental probability distribution for the class of conti- nuous functions is the classical Gaussian distribution named after the German Mathematician Karl Friedrich Gauss in 1809. Definition 1.1 Let µ and σ be constants with −∞ <µ < ∞ and σ > 0 . The function 2 1x−µ 1 − fx( ;,µσ) = e2σ ; for−∞ <x < ∞ (1) 2πσ 2 is called the normal probability density function of a random variable X with parameters µ and σ . Both in theories and applications, without element of equivocation, the Gaus- sian distribution function is the most essential and widely referencing distribu- DOI: 10.4236/am.2020.116031 Jun. 1, 2020 436 Applied Mathematics A. T. Adeniran et al. tion in statistics. The well-known method of deriving this distribution first appeared in the second edition of the Doctrine of Chances by Abraham de Moivre (hence, de Moivre’s Laplace limit theorem) published in 1738 ([1] [2] [3] [4] [5]). The mathematical statement of the popular de Moivre’s theorem follows. Theorem 1.1 (de Moivre’s Laplace limit theorem) As n grows large ( n →∞), for x in the neighborhood of np, for moderate values of p ( p ≠ 0 and p ≠ 1), we can approximate ( x− np)2 − n x nx− 1 2npq pq ≈ e ,p+= q 1, pq , > 0. (2) x 2πnpq + Explicitly, the theorem asserts that suppose n ∈ , and let p and q be proba- bilities, with pq+=1. The function n x nx− b( xnp;, ) =−= p(1 p) for x 0,1,2, , n (3) x called the binomial probability function converges to the probability density function of the normal distribution as n →∞ with mean np and standard deviation np(1− p) . 1 Although, De Moivre proved the result for p = ([6] [7]). [8] extended and 2 generalized the proof to all values of p (probability of success in any trial) such that p is not too small and not too big. Feller result was expounded by [9]. [10] [11] [12] [13] used uniqueness property of moment generating function tech- nique to proof the same theorem. In this paper, we attempt to find an answer to the question: is there any alter- native procedure to the derivation of Gaussian probability density function apart from de Moivre’s Laplace limit theorem approach which relies heavily on many Lemmas and Theorems (Stirling approximation formula, Maclaurin series ex- pansion etc.), as evidenced by the work of [8] and [9]? 2. Existing Technique This section presents the summary proof of the existing de Moivre’s Laplace limit theorem. First and foremost, the study state with proof, the most important lemma of the de-Moivre’s Laplace limit theorem, Stirling approximation prin- ciple. Lemma 2.1 (Stirling Approximation Principle) Given an integer nn;0> , the factorial of a large number n can be replaced with the approximation n n nn!2≈π e Proof 2.1 This lemma can be derived using the integral definition of the fac- torial, ∞ nx− n!=Γ+=( n 1) xx ed (4) ∫0 Note that the derivative of the logarithm of the integrand can be written DOI: 10.4236/am.2020.116031 437 Applied Mathematics A. T. Adeniran et al. dd− n ln( xnx e) =( n ln xx −=−) 1 (5) ddxxx The integrand is sharply peaked with the contribution important only near xn= . Therefore, let xn= +δ where δ ≤ n , and write nx− δ ln( x e) = nn ln ( +−+δδ) ( n) = nnln 1 + −+( nδ) n (6) δ =nnln( ) + ln 1 + −+(nδ ) n 1 Recall that the Maclaurin series of fx( ) =ln( 1 +=− x) x x2 +0( n) . There- 2 fore, δδ1 22δ nx− = + − + − +δ =n −− + ln( x e) n ln ( n) 2 (n) ln ( nn) (7) nn22n Taking the exponential on both sides of the preceding Equation (7) gives δ2 δδ 22n ln nnn −− n − − nx−−( ) ln(n ) n n x e≈= e2n e ee 22 nn = e (8) e Plugging (8) into the integral expression for n!, that is, (4) gives nnδδ22 ∞∞nn−− n!≈= ed22nnδδ ed (9) ∫∫0 ee −∞ δ 2 ∞ − From (9), let I = ed2n δ and considering δ and κ as a dummy varia- ∫0 ble such that δκ22 1 ∞−∞ − ∞∞−+δκ22 2 22α nn 2( ) I =×=e dδ e d κ e ddδκ (10) ∫0 ∫0 ∫∫00 Transforming I 2 from algebra to polar coordinates yields δρ= cos( θ) , κρ= sin ( θ) which implies δκ22+= ρ 2 with Jacobian (J) of the transforma- tion as ∂∂δδ ∂∂ρθcos(θ) − ρθ sin ( ) J = = = ρ (11) ∂∂κκsin(θρ) cos( θ) ∂∂ρθ Hence, ρρ22 22π∞−−π∞ IJ2 = e22nn ddρθ= e ρρθ dd ∫∫00 ∫∫00 (12) 22ππ∞ =−==ne−u dθθ nn d2π ∫∫000 Therefore, II= =2 π n. Substituting for I in (9) gives n n nn!2≈π (13) e We now begin with proof of theorem (1.1) using the popular existing tech- nique. DOI: 10.4236/am.2020.116031 438 Applied Mathematics A. T. Adeniran et al. Proof 2.2 Using the result of lemma (2.1), Equation (3) can be rewritten as n n 2πn e nx− ≈− x f( xnp;, ) x nx− p(1 p) x nx− 22πx π−( nx) ee (14) 1 n+ 2 1 n nx− = ppx (1− ) 1 1 2π x+ nx−+ x2 ( nx− ) 2 1 Multiplying both numerator and denominator of Equation (14) by nn≡ 2 to get 11 n+++− xx 22 1 n nx− f( xnp;, ) ≈−px (1 p) 1 1 2πn x+ nx−+ x2 nx− 2 ( ) (15) 11 −−x −+−nx 22 1 x nx− x nx− = pp(1− ) 2πn nn Since x is in the neighborhood of np, change variables x= np + ε , where ε measures the distance from the mean, np, of the binomial and the measured quantity x. Re-write (15) in terms of ε and further simplify as follow 11 −−x −+−nx 22 1 np+εε n −− np x nx− f( xnp;, ) ≈− p(1 p) 2πn nn 1 1 −−x −+−nx 2 2 1 εεx nx− ≈p1 +( 11 −−p)pp(1−) 2πn np n(1− p) 1 1 −−x −+−nx 2 1 1 εε2 − − 1 2 ≈+11 − pp( 1−) 2 2πn np n(1− p) to get 1 1 −−x −+−nx 1 εε2 2 f( xnp;, ) ≈1 +− 1 (16) 21π−np( p) np n(1− p) Note that xx= exp( ln ) . Therefore, rewriting (16) in exponential form to have 1 1 −−x −+−nx 1 εε2 2 f( xnp;, ) ≈ exp ln 1+− 1 − 21π−np( p) np n(1 p) 11εε 1 ≈ exp−−x ln 1 + +−+ nx − ln 1 − 21π−np( p) 2np 21 n( − p) 11ε f( x;, n p) ≈ exp −np −−ε ln 1 + 21π−np( p) 2 np (17) 1 ε +−np(1 −) +ε −ln 1 − 21np( − ) DOI: 10.4236/am.2020.116031 439 Applied Mathematics A. T. Adeniran et al. 1 Suppose fx( ) =ln( 1 + x) , using Maclaurin series fx( ) =−+ x x2 0( n) 2 1 and similarly fx( ) =ln( 1 − x) =−− x x2 +0( n) . So that, 2 2 2 εε 1 ε εεε 1 ln 1+ ≈− and ln 1− ≈− − . np np2 np np(1−−−) np( 1) 21 np( ) As a result, f( xnp;, ) 1 εε22 ε 2 ε 2 ≈ exp −+εε − + + − (18) 21π−np( p) 2np np 21 n( −− p) n( 1 p) 11ε 2 = exp − 21π−np( p) 21np( − p) 2 Recall that x= np + ε which implies that ε 2 =( x − np) . From binomial dis- tribution np = µ , and np(1−= p) σ 2 which implies that np(1−= p) σ . Making appropriate substitution of these in the Equation (18) yields 2 2 1x−µ 11( x − µ ) 1− f( xnp;, ) ≈ exp − = e2σ ; for −∞<x <∞ (19) 2 2 2πσ 2 σ σ 2π The theorem confirmed. We recommend that readers interested in the detailed proof of the theorem to consult the study expounded by [9]. 3. The Proposed Technique Suppose a random experiment of throwing needle or any other dart related ob- jects at the origin of the cartesian plane is performed with the aim of hitting the centre (see Figure 1). Due to human nature of inconsistency or lack of perfection, varying results in the throwing generate random errors. To make the derivation possible and less rigorous, we make the following assumptions: 1) The errors are independent of the orientation of the coordinate system.